1. Field of the Invention
The present invention relates to a semiconductor memory device, e.g., a dynamic random access memory (DRAM), having a data holding mode using an ECC function.
2. Description of the Related Art
A conventionally known example of a semiconductor memory device of this type is “a data holding method of a dynamic RAM and a semiconductor integrated circuit device” disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2002-56671. This patent reference has proposed a method of correcting error bits as follows at the start of an operation mode in which a DRAM performs only a data holding operation. That is, when this operation mode starts, parity bits (or check bits) for detecting and correcting errors are generated and stored for a plurality of data, and refreshing is performed by extending the refresh period within the error occurrence allowable range of an error correcting operation using the parity bits. Before the data holding operation returns to a normal operation, error bits are corrected by using the data and parity bits.
FIG. 23 of the patent reference described above is a timing chart showing timings from the end of an entry operation to the start of the data holding mode, and timings from the data holding mode to the start of an exit operation. In this method, reading of data from all memory cells and write of generated parity bits is performed in an entry period during which the normal operation changes to the data holding mode. Also, in an exit period during which the data holding mode changes to the normal operation, codes, i.e., information bits and the parity bits are read out from all the memory cells, errors are detected and corrected, and the corrected data is rewritten.
In the semiconductor memory device having the above arrangement, however, the transition time from the normal operation to the data holding mode or the transition time from the data holding mode to the normal operation is longer than the refresh time, so a row which is not accessed within the refresh time may exist. In addition, when the ECC function is to be used by a column-address-priority page operation, refreshing must be performed by interrupting the page operation which is accessing all cells, in order to satisfy the limitation of the refresh time. This makes control difficult.